Compositions And Methods for Pickling and Passivation of Stainless Steel Welds

ABSTRACT

Compositions and methods for simultaneous pickling and passivation of stainless steel are presented that have exceptional shelf life, are free of hydrofluoric acid, exhibit no syneresis, and are non-corrosive to carbon steel. Moreover, contemplated compositions can be safely sprayed or otherwise applied onto various surfaces and will adhere to treated surfaces without significant runoff.

This application claims priority to our U.S. Provisional Patentapplication with the Ser. No. 63/321,053, which was filed Mar. 17, 2022,and which is incorporated by reference herein.

FIELD OF THE INVENTION

The field of the invention is composition and methods for pickling andpassivation of stainless steel welds, especially as it relates tonon-corrosive and hydrogen fluoride (HF) free compositions.

BACKGROUND OF THE INVENTION

The background description includes information that may be useful inunderstanding the present invention. It is not an admission that any ofthe information provided herein is prior art or relevant to thepresently claimed invention, or that any publication specifically orimplicitly referenced is prior art.

All publications and patent applications herein are incorporated byreference to the same extent as if each individual publication or patentapplication were specifically and individually indicated to beincorporated by reference. Where a definition or use of a term in anincorporated reference is inconsistent or contrary to the definition ofthat term provided herein, the definition of that term provided hereinapplies and the definition of that term in the reference does not apply.

Heat tint is commonly observed in furnace treated stainless steel and inthe heat affected zone of weldments, and is the result of the thickeningof a naturally occurring transparent oxide layer on the surface ofstainless steel. The colors formed are similar to the “temper colors”seen on other steel surfaces following heat treatment and range frompale straw hues to dark blue. As heat tints are formed on the surface ofstainless steel, chromium is drawn to the surface due to itspreferential susceptibility to oxidation relative to iron in the steel.This leaves a layer at and just below the surface with a lower chromiumlevel than in the bulk of the steel, and consequently a surface withgreatly diminished corrosion resistance. Other oxides present in theheat tint include ferrite that can initiate accelerated corrosion of theweld area.

Heat tint on stainless steel fabrications can be removed using acidicbrush-on pastes or gels, spray pickling, or immersion tank pickling.Pickling is a metal surface treatment that removes impurities, such asrust or scale from metals. Most commonly, hydrofluoric acid is the basecomponent that is used for the pickling of stainless steel.Unfortunately, products containing this harmful and toxic acid requirespecial handling procedures, and its fumes are dangerous to the user andenvironment. For at least this reason, hydrofluoric acid containingpickling compositions are unsuitable for spray application as suchapplication creates a highly toxic aerosol to a user. Passivation is ametal surface treatment in which a light coat of a protective materialis applied on the metal surface to create a barrier against corrosion.This is typically achieved by nitric acid, which is somewhat easier tohandle and by itself is a less dangerous acid.

For example, U.S. Pat. No. 6,844,304 to Lunner describes a compositioncomprising nitric acid, urea, and a filler to remove an oxide layer ofstainless steel after heat treatment. Nevertheless, and especially incombination with hydrofluoric acid, such pickling and passivationcompositions are still problematic in use and disposal. In otherattempts to remove heat tint, as described in U.S. Pat. No. 2,765,271 toKreml, discoloration is removed by electrolytic treatment of the weldarea using phosphoric acid. Similarly, and as disclosed in WO2013/036999 to Lewer, the heat tint is electrochemically removed usinglow voltage high amperage current with an electrolytic fluid comprisinga potassium phosphate salt of an acid at neutral pH. While suchcompositions are generally benign to the environment and operator, useis complicated by the need for specific equipment and electricity.

Advantageously, pickling and passivation can be performed using a singlecomposition, and U.S. Pat. No. 10,280,515 describes a method in whichmagnesium salts of hydrofluoric and nitric acid are used in an aqueousbase. Here, magnesium nitrate and magnesium fluoride are present in aratio of about 2:1 and are used to help prevent dissociation ofmagnesium fluoride that would otherwise lead to generation ofhydrofluoric acid. Unfortunately, the production of magnesium nitrate isa highly exothermic process and tends to complicate manufacture.Moreover, at least some of such formulations have a shelf life that isless than desirable.

Thus, even though various compositions and methods of pickling andpassivating stainless steel welds are known in the art, all or almostall of them suffer from several drawbacks, particularly where thecompositions contain hydrofluoric acid. Therefore, there remains a needfor improved hydrofluoric acid-free and non-corrosive pickling andpassivating compositions that are environmentally friendly and safe toapply to a variety of surfaces.

SUMMARY OF THE INVENTION

The inventive subject matter is directed to various compositions andmethods for simultaneous pickling and passivation of stainless steelthat have exceptional shelf life, that are free of hydrofluoric acid,exhibit no syneresis, and that are non-corrosive to carbon steel.

In one aspect of the inventive subject matter, the inventor contemplatesa pickling and passivating composition that includes a magnesiumfluoride salt, and nitric acid and/or a nitrate salt in an aqueousmedium, and that further includes a water-swellable crosslinkedcopolymer in an amount sufficient to thicken the composition to allowfor the composition to cling to a vertical surface. Most preferably, thecrosslinked copolymer is chemically stable in the composition such that,upon storage for at least one year, the composition exhibitssubstantially no syneresis and exhibits a loss in viscosity of no morethan 5%.

For example, suitable water-swellable crosslinked copolymers are inverseemulsion polymerization copolymers, which may be prepared from anacrylamide, an acrylamidoalkyl-sulfonic acid, and/or a salt thereof, andan amine-based polyfunctional crosslinking agent (e.g., Solagum SH210).In further examples, the composition may further comprise sulfamic acidand/or calcined alumina. As will be readily appreciated, the magnesiumfluoride salt, the nitric acid, and the nitrate salt are formed in situby reaction of magnesium nitrate hexahydrate and hydrofluoric acid.

In further contemplated embodiments, the copolymer is present in anamount of at least 2.5 wt %, and/or the loss in viscosity is no morethan 3%. Where desired, the composition can be packaged into a multi-usetube or multi-use container, or a container from which the compositioncan be drawn to be sprayed onto a surface. Therefore, contemplatedcompositions may be provided in association with an instruction to spraythe composition to a surface.

Viewed from a different perspective, the inventor also contemplates amethod of preparing a pickling and passivating composition that includesa step of dissolving magnesium nitrate hexahydrate in an aqueous mediumto form a magnesium nitrate solution, and a further step of addinghydrofluoric acid to the magnesium nitrate solution to form a magnesiumfluoride precipitate and nitric acid. In another step, a water-swellablecrosslinked copolymer is added in an amount that is sufficient tothicken the composition to allow for the composition to cling to avertical surface. Most preferably, the crosslinked copolymer ischemically stable in the composition such that, upon storage for atleast one year, the composition exhibits substantially no syneresis andexhibits a loss in viscosity of no more than 5%.

Among other options, the water-swellable crosslinked copolymer is aninverse emulsion polymerization copolymer. For example, suitablecopolymers may be prepared from an acrylamide, anacrylamidoalkyl-sulfonic acid, and/or a salt thereof, and an amine-basedpolyfunctional crosslinking agent. Thus, exemplary copolymers includeSolagum SH210. In addition, it is contemplated that sulfamic acid and/orcalcined alumina may be added to the magnesium nitrate solution, and/orthat the nitric acid reacts with the sulfamic acid to form NO₂ andsulfuric acid.

In some embodiments, the copolymer is added in an amount of at least 2.5wt % of the composition, the magnesium nitrate hexahydrate is dissolvedin an amount of about 43 wt % of the composition, and/or thehydrofluoric acid is added in an amount of about 9 wt % of thecomposition. In further embodiments, the water-swellable crosslinkedcopolymer is added in an amount of about 3 wt % of the composition.

Therefore, the inventor also contemplates a method of pickling andpassivating a stainless steel surface in which the compositionspresented herein are applied to the stainless steel surface for a timesufficient to pickle and passivate the stainless steel surface. Mosttypically, the stainless steel surface comprises a weld, and/or thecomposition is applied by spraying or from a multi-use container.

Various objects, features, aspects, and advantages of the inventivesubject matter will become more apparent from the following detaileddescription of preferred embodiments, along with the accompanyingdrawing figures in which like numerals represent like components.

DETAILED DESCRIPTION

The inventor has discovered various compositions and methods forsimultaneous pickling and passivation of stainless steel that haveexceptional shelf life, that are free of hydrofluoric acid, exhibit nosyneresis, and that are non-corrosive to carbon steel. In furtherbeneficial aspects, the compositions can be sprayed without adverseeffects on the operator and will cling to the treated surface withoutsignificant runoff

In prior formulations, magnesium oxide was reacted with nitric acid toform magnesium nitrate in a highly exothermic process. The so producedmagnesium nitrate was then reacted with hydrofluoric acid to generatemagnesium fluoride and nitric acid, and the magnesium fluoride was thenused as a pickling agent while the nitric acid was used as a passivatingagent. As should be readily apparent, such production method wasdifficult to control and environmentally problematic. Moreover, whilesuch formulations could be brushed onto a surface, cling to verticalsurfaces was at least in some cases problematic.

In contrast, in one exemplary production process according to theinventive subject matter, magnesium nitrate hexahydrate is dissolved inwater to a concentration of about 43.03% (w/w). Finely ground calcinedalumina is added as a colorant in an amount of about 4.31% (w/w), andsulfamic acid is added in an amount of about 2.94% (w/w). Hydrofluoricacid is added to the so prepared mixture in an amount of about 9.7%(w/w), which will react with magnesium nitrate, thereby forming amagnesium fluoride precipitate and nitric acid. Some of the nitric acidin turn reacts with sulfamic acid to produce NO₂ and sulfuric acid. Tothis mixture is then added about 2-3% (w/v) of a synthetic acrylatepolymer that is inert to and long-term stable in acidic conditions inthe composition. Preferably, the synthetic polymer is made by inverseemulsion polymerization (e.g., using an acrylamide, anacrylamidoalkyl-sulfonic acid, and/or a salt thereof and an amine-basedpolyfunctional crosslinking agent) and is used as an emulsifyingthickener to produce a stable composition with gel-like consistency.

Notably, the inventor has discovered that such polymers, when added inproper quantities and appropriate ratio to MgNO₃/HF (e.g.,water-swellable crosslinked copolymers, emulsifying thickeners), had theunexpected effect to stabilize contemplated formulations oversignificant periods of time even in the absence of a hydrophobic phase.Among other benefits, such polymers produced thickened formulationswithout syneresis and so enabled extended storage and usability withoutany degradation and loss in viscosity. While conventional viscosityagents will contract and expel water under the acidic conditions andcannot be recombined by subsequent stirring/mixing (in a manner similarto coagulated blood that cannot be ‘un-coagulated’ by stirring/mixing),the compositions presented herein can be formulated to any desiredthickness and will not separate into a contracted polymeric phase and anexpelled water phase.

Suitable polymers therefore include water-swellable crosslinkedcopolymers that are chemically stable at the pH and in the compositionspresented herein. Viewed from a different perspective, suitable polymerswill have a drop in viscosity of no more than 15%, or no more than 10%or no more than 8%, or no more than 6%, or no more than 4%, or no morethan 3%, or no more than 2%, or no more than 1% after storage of atleast 6 months, or at least 12 months, or at least 18 months, or atleast 24 months, or at least 36 months with a storage temperature ofabout 20-25° C. Likewise, suitable polymers will exhibit no (less than1% of water expelled and separable from the composition) orsubstantially no (less than 5% of water expelled and separable from thecomposition) syneresis after storage of at least 6 months, or at least12 months, or at least 18 months, or at least 24 months, or at least 36months with a storage temperature of about 20-25° C. Exemplary polymerssuitable for use are described in U.S. Pat. Nos. 8,936,797, 6,683,144,FR 2,943,677, and EP 1 116 733 all of which are incorporated byreference herein (e.g., selected polymers are commercially availablefrom SEPPIC S.A.). In addition to SEPPIC-type water swellable polymers,poly(ethylene oxide) based polymers such as Polyox N 3000 (commerciallyavailable from DuPont) can also be used, individually or in combinationwith SEPPIC-type polymers. In contrast, conventional viscosity modifierssuch as various gums, cellulosics, pectins, and polyvinyl alcohols haveshown little to no stability under the acid conditions of theformulations presented and have led to significant reduction inviscosity and syneresis. As such, these conventional viscosity modifiersdo not allow for long-term storage.

It should further be appreciated that where a composition is preparedfrom hydrofluoric acid and magnesium nitrate, all or substantially all(e.g., at least 95 mol %, more typically at least 98 mol %) of thehydrofluoric acid has reacted with magnesium nitrate to so form amixture of magnesium nitrate and magnesium fluoride salts where themagnesium fluoride is very little or insoluble in water. Without wishingto be bound by any theory or hypothesis, the inventor contemplates thatsuch mixture can be obtained by combination of magnesium nitrate inmolar excess with hydrofluoric acid. Viewed from a differentperspective, molar excess of magnesium nitrate will ensure thatformation of hydrogen fluoride will not take place or is greatlyreduced, therefore rendering the composition to be free or substantiallyfree (e.g., at least 5 mol %, more typically at least 2 mol %, mosttypically less than 1 mol %) of hydrofluoric acid. In most typicalembodiments, a composition that is substantially free of hydrofluoricacid comprises an amount of less than 5 ppm (parts per million)hydrofluoric acid, and more preferably less than 2 ppm hydrofluoricacid.

Most typically, the pH of the composition will be equal of less than6.0, or equal of less than 5.5, or equal of less than 5.0, or equal ofless than 4.5, or equal of less than 4.0, or equal of less than 3.5, orequal of less than 3.0, or equal of less than 2.5, or equal of less than2.0, or even more acidic. Contemplated compositions can further includesulfamic acid or oxalic acid in an amount between 0.5-50% by weight ofthe composition, and more typically between 1-15% by weight, and mosttypically 2-5% by weight of the composition. The sulfamic acid or oxalicacid further ensures that the magnesium fluoride salt does notdissociate. When there is a large amount of dilution, there is a riskthat the magnesium fluoride salt can dissociate, and the fluoride ion isused to produce hydrofluoric acid. However, using sulfamic acid oroxalic acid acts as a barrier and creates protons to prevent magnesiumfluoride salt from dissociating even when there is a large amount ofdilution. In addition, the sulfamic acid or oxalic acid acts as achelating agent to remove iron oxide from stainless steel, such that theiron oxide dissolves so that it can be rinsed off the stainless steel.Therefore, the composition can further be used to remove rust fromsteel. Most notably, contemplated compositions were shown to benon-corrosive, even when used on carbon steel.

Contemplated compositions and formulations may further includeadditional ingredients to provide one or more desired functionalities,and particularly preferred additional ingredients include surfactants,chelators, fillers, pigments, and odor masking agents. Notably, as thepreferred polymers also act as emulsifying thickener, surfactants may beomitted. However, it should be noted that surfactants not excluded andmay be present in an amount of between 0.5-15% by weight of thecomposition, and more typically between 0.5-5% by weight of thecomposition. Suitable surfactants for the composition include at leastone of a linear alkyl benzene sulphonic acid and alcohol ethoxylates(C₉₋₁₁).

Chelators can be included in the composition between 1.0-15% by weightof the composition, and more typically between 1.0-5% by weight of thecomposition. Suitable chelators include one or more mono-, bi, andpolydentate chelators. For example, contemplated chelators includegluconic acid, citric acid, tartaric acid, gluconate salts, andethylenediaminetetraacetic acid (EDTA). While not limiting to theinventive subject matter, chelating agents are generally preferred incontemplated formulations as chelating agents assist with the removal ofpickled oxides of chromium and iron.

It should be appreciated that regardless of the type of formulation, thecomposition suitable for treating heat tint by pickling and passivationof stainless steel can be applied in cold temperatures. It iscontemplated that the composition can include an antifreeze componentthat does not create auxiliary reactions. Typically, the antifreezecomponent is propylene glycol. The addition of the antifreeze componentallows the composition to be applied in temperatures below 0° C. andeven below −20° C. or −50° C. Advantageously, the composition can beapplied in gel, paste, or spray formulation throughout the year withoutincident. The antifreeze component can be included in an amount between2-30% by weight of the composition, and more typically between 10-20% byweight of the composition. Other suitable antifreeze components includemethanol or a glycol (e.g., ethylene glycol).

In further aspects of the inventive subject matter, a method of treatinga heat tint by pickling while simultaneously passivating a stainlesssteel is contemplated. The method includes a step of contacting thestainless steel with a formulation that comprises a magnesium fluoridesalt, a nitrate salt, an acid, and an acrylate copolymer such that thenitrate salt and the acid are present in an amount sufficient to preventformation of hydrofluoric acid caused by dissociation of the magnesiumfluoride salt, and such that the acrylate copolymer provides sufficientthickening to maintain the formulation on the treated surface for anextended time. Contemplated acids that prevent formation of hydrofluoricacid include oxalic acid and sulfamic acid.

When the formulation is applied to a heat tint, the stainless steel ispickled to remove the heat tint while the steel is simultaneouslypassivated to protect it from corrosion. It should be appreciated thatthe magnesium fluoride salt is present in an amount effective to reduceor remove weld marks by pickling while the nitrate salt is present in anamount effective to promote passivation of the stainless steel.Therefore, the stainless steel is the heat tint is removed and thestainless steel is protected by passivation using the formulation.

After applying the formulation and allowing time for pickling andpassivation of the stainless steel, the formulation is removed from thestainless steel. Contemplated contact times for the formulation onto thestainless steel are between 1-4 hours. The formulation can be removed byusing pressurized water. However, since the formulation will beextremely diluted by the addition of pressurized water, there can be aproblem with the dissociation of the magnesium fluoride salt. Suchdissociation can result in the formation of hydrofluoric acid using thefluoride ion of the dissociated magnesium fluoride salt, which is atoxic residue harmful to the environment. Advantageously, using anitrate salt and an acid (e.g., oxalic acid or sulfamic acid) asdescribed herein prevented or substantially reduces the formation ofhydrofluoric acid by reducing the likelihood that the magnesium fluoridesalt will dissociate.

It should be appreciated that the formulation can also be applied torust on metals. Thus, the formulation can be used in areas of the steelthat are not affected by a heat tint. Indeed, the formulation describedherein comprising oxalic acid or sulfamic acid will act as a chelatingagent dissolving the iron oxide so that it could easily be removed.

In the context of application of the formulations, it should beappreciated that the formulations presented herein cannot only beprepared as a liquid, but also as a brushable or sprayable formulationthat has sufficient cling. For example, exemplary compositions withdesirable cling will remain on a vertical surface such that afterapplication of a quantity of the composition at least 75%, or at least80%, or at least 85%, or at least 90%, or at least 95%, or at least 98%of the applied quantity will remain on a vertical surface where thesurface is steel and where the applied composition has an averagethickness of at least 0.5 mm, or at least 0.7 mm, or at least 0.9 mm, orat least 1.2 mm, or at least 1.5 mm, or at least 2 mm, or at least 3 mm,or at least 4 mm, and even thicker.

Moreover, due to their exceptional stability, it is contemplated thatthe formulations may be stored in single- or multi-use tubes or othercontainers at relatively small volume (e.g., less than 1,000 mL) fromwhich the formulation can be directly dispensed to the surface to betreated (e.g., by use of a tube with applicator tip or a caulking gun).This will advantageously allow portability of the formulations to thepoint of use in a tool box or other container and avoid the need forlarge containers and/or on-site preparation. Indeed, contemplatedformulations have shown remarkable storage stability and maintainedsubstantially identical working parameters for at least 3 years.However, storage in larger containers is also contemplated, and suchcontainers may be portable (e.g., enclosing a volume of up to 5,000 mL,or up to 10 L, or up to 20 L), or in a larger form factor for on-site orwhole sale storage from which portions can be dispensed over an extendedperiod (e.g., at least 1 year, or at least 2 years, etc.)

In still further contemplated embodiments, the compositions presentedherein can also be formulated to avoid freezing, and even be effectiveat temperatures as low as minus 40 degrees Celsius. This can be achievedby replacing 50% or less (w/w) of the amount of water in any of thegiven variations of the formulation with an antifreeze agent. Amongother suitable choices, preferred antifreeze agents include propyleneglycol due to its non-toxic and biodegradable nature. Alternatively,further antifreeze agents can be used, such as ethylene glycol, and thewater to antifreeze ratio will generally be determined by the azeotropiccurve of the mixture of the two components as a function of the freezingpoint value.

EXAMPLES

The following experimental data is disclosed to provide exemplaryformulas that achieve pickling and passivation of stainless steel byusing non-water soluble salts under conditions that lack of freehydrofluoric acid.

In particular, the inventor prepared a variety of formulations as shownin Table 1 below and then tested pickling and passivation performancealong with measurements in drop in viscosity and ability to cling tostainless steel at a 90° angle to normal. Performance indicated innumeric values with 5 being best and 0 being worst.

TABLE 1 Ingredient (wt %/wt %) A B C D Magnesium Nitrate × 6H₂O 43.0338.00 43.03 43.03 HF (49%) 9.74 8.60 9.74 9.74 Sulfamic Acid 2.94 2.942.94 2.94 Alumina Calcinated 4.31 4.31 4.31 4.31 SolaGum SH210 2.94 2.941.5 N/A Polyox WSR N3000 N/A N/A N/A 6.00 Water Balance Balance BalanceBalance Pickling/Passivation 5 3 4 4 Performance Drop in viscosity (12months) 1-3% 1-3% 30-40% 40-60% 90 Degrees Cling to SS panel 5 5 2 1Syneresis none visible visible visible

As can be seen from the table, most formulations had a desirablepickling/passivation performance. Notably, cling to a vertical surfacewas superior with water-swellable crosslinked copolymers/emulsifyingthickeners, and desirable stability of viscosity was achieved when thewater-swellable crosslinked copolymers, emulsifying thickeners waspresent in an amount of more than 1.5 wt %. The physicochemicalproperties were even further enhanced where MgNO₃/HF and thewater-swellable crosslinked copolymers/emulsifying thickeners had aratio of at least 16.5, and more typically at least 17, or at least17.5, or at least 18.0.

In some embodiments, the numbers expressing quantities of ingredients,properties such as concentration, reaction conditions, and so forth,used to describe and claim certain embodiments of the invention are tobe understood as being modified in some instances by the term “about.”As used herein, the terms “about” and “approximately”, when referring toa specified, measurable value (such as a parameter, an amount, atemporal duration, and the like), is meant to encompass the specifiedvalue and variations of and from the specified value, such as variationsof +/−10% or less, alternatively +/−5% or less, alternatively +/−1% orless, alternatively +/−0.1% or less of and from the specified value,insofar as such variations are appropriate to perform in the disclosedembodiments. Thus, the value to which the modifier “about” or“approximately” refers is itself also specifically disclosed. Therecitation of ranges of values herein is merely intended to serve as ashorthand method of referring individually to each separate valuefalling within the range. Unless otherwise indicated herein, eachindividual value is incorporated into the specification as if it wereindividually recited herein.

All methods described herein can be performed in any suitable orderunless otherwise indicated herein or otherwise clearly contradicted bycontext. The use of any and all examples, or exemplary language (e.g.,“such as”) provided with respect to certain embodiments herein isintended merely to better illuminate the invention and does not pose alimitation on the scope of the invention otherwise claimed. No languagein the specification should be construed as indicating any non-claimedelement essential to the practice of the invention.

As used in the description herein and throughout the claims that follow,the meaning of “a,” “an,” and “the” includes plural reference unless thecontext clearly dictates otherwise. Also, as used in the descriptionherein, the meaning of “in” includes “in” and “on” unless the contextclearly dictates otherwise. As also used herein, and unless the contextdictates otherwise, the term “coupled to” is intended to include bothdirect coupling (in which two elements that are coupled to each othercontact each other) and indirect coupling (in which at least oneadditional element is located between the two elements). Therefore, theterms “coupled to” and “coupled with” are used synonymously.

It should be apparent to those skilled in the art that many moremodifications besides those already described are possible withoutdeparting from the inventive concepts herein. The inventive subjectmatter, therefore, is not to be restricted except in the scope of theappended claims. Moreover, in interpreting both the specification andthe claims, all terms should be interpreted in the broadest possiblemanner consistent with the context. In particular, the terms “comprises”and “comprising” should be interpreted as referring to elements,components, or steps in a non-exclusive manner, indicating that thereferenced elements, components, or steps may be present, or utilized,or combined with other elements, components, or steps that are notexpressly referenced. Where the specification or claims refer to atleast one of something selected from the group consisting of A, B, C . .. and N, the text should be interpreted as requiring only one elementfrom the group, not A plus N, or B plus N, etc.

What is claimed is:
 1. A pickling and passivating composition,comprising: a magnesium fluoride salt, and nitric acid and/or a nitratesalt in an aqueous medium; a water-swellable crosslinked copolymer in anamount sufficient to thicken the composition to allow for thecomposition to cling to a vertical surface; and wherein the crosslinkedcopolymer is chemically stable in the composition such that, uponstorage for at least one year, the composition exhibits substantially nosyneresis and exhibits a loss in viscosity of no more than 5%.
 2. Thecomposition of claim 1, wherein the water-swellable crosslinkedcopolymer is an inverse emulsion polymerization copolymer.
 3. Thecomposition of claim 2, wherein the copolymer is prepared from anacrylamide, an acrylamidoalkyl-sulfonic acid, and/or a salt thereof, andan amine-based polyfunctional crosslinking agent.
 4. The composition ofclaim 3, wherein the copolymer is Solagum SH210.
 5. The composition ofclaim 1, wherein the composition further comprises sulfamic acid and/orcalcined alumina.
 6. The composition of claim 1, wherein the magnesiumfluoride salt, and nitric acid and/or a nitrate salt are formed in situby reaction of magnesium nitrate hexahydrate and hydrofluoric acid. 7.The composition of claim 1, wherein the copolymer is present in anamount of at least 2.5 wt %.
 8. The composition of claim 1, wherein theloss in viscosity is no more than 3%.
 9. The composition of claim 1,wherein the composition is packaged into a multi-use tube or multi-usecontainer.
 10. The composition of claim 1 in association with aninstruction to spray the composition to a surface.
 11. A method ofpreparing a pickling and passivating composition, comprising: dissolvingmagnesium nitrate hexahydrate in an aqueous medium to form a magnesiumnitrate solution; adding hydrofluoric acid to the magnesium nitratesolution to form a magnesium fluoride precipitate and nitric acid; andadding a water-swellable crosslinked copolymer in an amount sufficientto thicken the composition to allow for the composition to cling to avertical surface; wherein the crosslinked copolymer is chemically stablein the composition such that, upon storage for at least one year, thecomposition exhibits substantially no syneresis and exhibits a loss inviscosity of no more than 5%.
 12. The method of claim 11, wherein thewater-swellable crosslinked copolymer is a copolymer is an inverseemulsion polymerization copolymer.
 13. The method of claim 12, whereinthe copolymer is prepared from an acrylamide, anacrylamidoalkyl-sulfonic acid, and/or a salt thereof, and an amine-basedpolyfunctional crosslinking agent.
 14. The method of claim 13, whereinthe copolymer is Solagum SH210.
 15. The method of claim 11, furthercomprising a step of adding sulfamic acid and/or calcined alumina to themagnesium nitrate solution, wherein the nitric acid reacts with thesulfamic acid to form NO₂ and sulfuric acid.
 16. The method of claim 11,wherein the copolymer is added in an amount of at least 2.5 wt % of thecomposition.
 17. The method of claim 11, wherein the magnesium nitratehexahydrate is dissolved in an amount of about 43 wt % of thecomposition, wherein the hydrofluoric acid is added in an amount ofabout 9 wt % of the composition, and/or wherein the water-swellablecrosslinked copolymer is added in an amount of about 3 wt % of thecomposition.
 18. A method of pickling and passivating a stainless steelsurface, comprising a step of applying the composition of claim 1 to thestainless steel surface for a time sufficient to pickle and passivatethe stainless steel surface.
 19. The method of claim 18, wherein thestainless steel surface comprises a weld.
 20. The method of claim 18,wherein the composition is applied by spraying or from a multi-usecontainer.